Some pyridoindole derivatives indicated by the following general formula (III) and salts thereof are publicly known in the Japanese Kokai Tokkyo Koho 2-117675. These pyridoindole derivatives and salts thereof possess a 5-HT antagonism and are effective in the treatment and the prevention of central nervous system (CNS) disorders such as psychosis (e.g., schizophrenia, mania, etc.), anxiety, depression and so forth; pains such as headaches (e.g., migraine headaches, cluster headaches, vascular headaches, etc.), neuralgia (e.g., trigeminal neuralgia, etc.) and so forth; gastrointestinal disorders such as symptoms of gastrointestinal dysfunction accompanying dyspepsia, peptic ulcer, reflux esophagitis, meteorism, and the like, irritable bowel syndrome (IBS), and the like; nausea or vomiting accompanying cancer treatment; motion sickness; and so forth. ##STR1## wherein R.sup.1 is hydrogen, a lower alkyl group or a lower alkenyl group, R.sup.2 is hydrogen, a lower alkyl group or a halogen, and R.sup.3 is an imidazolyl group which may have a suitable substituent (s).
A process for the synthesis of (.+-.)-8,9-dihydro-10-methyl-7-[(5-methyl-1H-imidazol-4-yl)methyl]pyrido[1 ,2-a]indol-6(7H)-one (IIIa) is described in Japanese Kokai Tokkyo Koho 2-117675 through the following 5 steps which are illustrated below specifically as an example of the process for the synthesis of pyridoindole derivatives (IIIa).
Conventional Route: ##STR2##
In the above described conventional processes, the object compound (IIIa) is obtained from the compound (Ia) by means of processes through five steps, which comprises an alkylation first being carried out through an aldol reaction with an aldehyde (V), followed by acetylation, deacetoxification, detosylation, and reduction to synthesize the compound (IIIa). Here, the yield from the compound (Ia) to the object compound (IIIa) is 68%. The processes for producing the pyridoindole derivatives shown by the general formula (III) which are disclosed in the aforesaid Japanese Tokkyo Koho have drawbacks in the complicated processes due to the large number of steps, in the low yield, and in the higher cost, and so forth.
Furthermore, the aforesaid pyridoindole derivative is generally obtained as a racemic mixture. Moreover, the stereoisomers of said pyridoindole derivatives are generally obtained by the resolution process involving a reaction with an optically active reagent such as, for example, diparatoluoyltartaric acid or the like. However, the conventional optical resolution processes using diparatoluoyltartaric acid or the like have the drawback not only in the low yield but also in the use of a large amount of the solvent for the crystallization, and also are inavoidable to use chloroform which is in trouble for its safety.